The present invention relates to an electromagnetic coupling for selectively transmitting power to a supercharger in an automobile, for example, and more particularly to such an electromagnetic coupling for use as an electromagnetic clutch.
Some recent high-grade automobiles employ superchargers which are selectively driven by electromagnetic clutches when the rotational speed of the engine reaches a certain speed level. The electromagnetic clutch comprises a rotor having excitation coils and an annular armature which is movable toward and away from the rotor. An outer holder plate having a cross-shaped flange is attached to the armature. An inner holder plate similar in shape to the outer holder plate is disposed in the outer holder plate with an elastomeric member such as cushioning rubber member being interposed between the inner and outer holder plates. The inner holder plate is coupled to the compressor wheel of the supercharger through a rotatable drive shaft.
In operation, the excitation coils are energized to magnetically attract the armature to the rotor, so that rotation of the rotor can be transmitted through the outer holder plate, the cushioning rubber member, the inner holder plate, and the rotatable drive shaft to the compressor wheel of the supercharger. FIGS. 7(a) and 7(b) of the accompanying drawings show the manner in which the electromagnetic clutch operates. When the rotor magnetically attracts the armature, the outer holder plate a is moved in the direction of the arrow A with respect to the inner holder plate b, as shown in FIG. 7(a). Therefore, the cushioning rubber member c is elastically deformed such that it is inclined downwardly (FIG. 7(b)) in the direction from the inner holder plate b toward the outer holder plate a.
A portion of the cushioning rubber member c which is close to the outer holder plate a is subjected to a greater amount of elastic deformation per unit area than the other portion of the cushioning rubber member c. The cushioning rubber member c thus deformed is then subjected to twisting forces when the rotor is rotated. Therefore, the elasticity constant K (Kgm/deg) of the cushioning rubber member c in the direction in which it is twisted is progressively reduced as the elastic deformation D (mm) of the cushioning rubber member c increases across a gap region G and a joining region C. Each time the electromagnetic coupling is connected or the torque of the supercharger is varied, the cushioning rubber member c is twisted to a large degree. As a consequence, the cushioning rubber member c tends to deteriorate due to fatique in a short period of time. Premature deterioration of the cushioning rubber member c results in an operation failure of the coupling and hence the supercharger connected to the coupling.
As shown in FIG. 7(b), when the outer holder plate a is moved toward the rotor, the cushioning rubber member c and the outer holder plate a form an acute angle R therebetween, causing stress concentration in a portion indicated by X. The stress concentration is apt to develop cracks in the cushioning rubber member c, with the result that the cushioning rubber member c will deteriorate soon.